Integrated Geophysical Exploration of a Known Geothermal Resource: Neal Hot Springs

摘要

I present an integrated geophysical characterization of the Neal Hot Springs geothermal system in eastern Oregon. This newly established 23 MW geothermal production site is located in a region of complex geology and intersecting faults associated with two major extensional systems; the Oregon-Idaho Graben and the western Snake River Plain. The intersection of two dominant fault orientations, coupled with a high geothermal gradient from relatively thin continental crust and radioactive decay from a shallow granitic body, produces the pathways and heat needed for deep water circulation at Neal Hot Springs. New geologic mapping, geochemistry, and measurements from several boreholes in the area suggest a normal fault dips to the southwest to form a half-graben basin. This basin-bounding fault, termed the Neal Fault, serves as the primary conduit for deep water circulation. Potential field, electrical, and seismic data characterize this major fault along with other structures in the area. A self-potential survey indicates that water is upwelling in the fault zone, and suggests that the fault provides the means for heated water to migrate to the surface. Electrical and magnetic surveys offer methods to locate hydrothermal waters within 0.5 km of ground surface and at the surface. Gravity models help integrate and refine my interpretations of the geologic structures. I then use tectonic faulting as a proxy for permeability to form a comprehensive three-dimensional subsurface permeability model of Neal Hot Springs. This integrative geophysical approach is not typical in geothermal exploration and offers to improve costs related to exploration in complex geologic environments.